1. Field of Invention
This invention relates to an optical microscope; and more particularly, to an improvement thereof which results in reduced background light and achieves high S/N ratio in scanning type or reflecting type confocal optical microscopes.
2. Description of the Prior Art
Scanning reflecting type confocal optical microscopes are well known. FIG. 1 shows an example of a conventional confocal optical microscope, wherein light beam emitted from a suitable light source l,such as a mercury arc lamp or laser, is polarized with a polarizer 2 into a linearly polarized light beam. The polarized light beam is transmitted through beam splitter 3 and is made incident to pinhole array disk 4, such as a Nipkow disk.
The light beam that is transmitted through pinhole array disk 4 is polarized into a circularly polarized light beam by a quarter wavelength plate 6 (also abbreviated .lambda./4 plate). The circularly polarized light beam is focused by objective lens 7 and irradiated on sample 8. In this case, a plurality of spots irradiated on sample 8 is sequentially scanned with the light beam polarized circularly by rotating pinhole array disk 4 by driving the disk 4 with motor 5.
The light beam reflected by sample 8 is focused by objective lens 7 and is linearly polarized by passing through the quarter wavelength plate 6, and forms an image on the same pinhole array disk 4. The light beam that is passed through the pinholes of disk 4 is made incident to beam splitter 3, reflected in the perpendicular direction, is made incident to analyzer 9, and forms an image on camera 11 via relay lens 10 after being polarized linearly. The image of sample 8 captured with camera 11 is displayed on the screen of monitor 12.
In the conventional prior art system, background light caused by pinhole array disk 4 can be reduced about 10.sup.-4 in light intensity by use of a pair of polarizers 2 and analyzer 9 and quarter wavelength plate 6. However, there is still the following problem. Quarter wavelength plate 6 is arranged between objective lens 7 and pinhole array disk 4 in order to shield the light beam incident on sample 8 from the light beam reflected from sample 8. In this system, the reflected light beam caused by pinhole array disk 4 can be cut off by analyzer 9. However, the light beam reflected from the surface of objective lens 7, after the incident light beam passes through quarter wavelength plate 6, cannot be cut off by analyzer 9 because objective lens 7,located under quarter wavelength plate 6, is handled in the same manner as sample 8 during polarization process.
Accordingly, a reflection preventing coat is applied on the surface of objective lens 7. But, the objective lens 7 has a reflectivity ranging from 0.5 to several percent. Such reflection scarcely causes the above problem if an object whose surface reflectivity is high, such as a metallic surface, is to be measured. However, if a sample is to be measured whose surface reflectivity is low, such as the inside of multiple layers, skin, or the inside of a living body, for example having a reflectivity lower than 0.1%, such reflection as background light significantly hinders observation and measurement. A similar problem exits even if a sample has a high surface reflectivity if it is greatly inclined, because the signal becomes very weak.